The invention relates generally to light emitting devices, and, more particularly, to a method for producing an InGaAsN active region for a long wavelength light emitting device.
Light emitting devices are used in many applications including optical communication systems. Optical communication systems have been in existence for some time and continue to increase in use due to the large amount of bandwidth available for transporting signals. Optical communication systems provide high bandwidth and superior speed and are suitable for efficiently communicating large amounts of voice and data over long distances. Optical communication systems that operate at relatively long wavelengths on the order of 1.3 micrometers (xcexcm) to 1.55 xcexcm are generally preferred because optical fibers generally have their lowest attenuation in this wavelength range. These long wavelength optical communication systems include a light source capable of emitting light at a relatively long wavelength. Such a light source is a vertical-cavity surface-emitting laser (VCSEL), although other types of light sources are also available.
The alloy indium gallium arsenide nitride (InGaAsN) is useful to form the active-region for a VCSEL operating at the long wavelengths preferred for optical fiber communication. This material allows the operating wavelength of a conventional aluminum gallium arsenide (AlGaAs) VCSEL to be extended to approximately 1.3 xcexcm. Furthermore, in other applications using photonic devices, such as light emitting diodes (LEDs), edge-emitting lasers, and vertical-cavity surface-emitting lasers (VCSELs), excellent performance characteristics are expected for InGaAsN active regions as a consequence of the strong electron confinement offered by AlGaAs heterostructures, which, provide carrier and optical confinement in both edge-emitting and surface-emitting devices. InGaAsN active regions benefit both edge- and surface-emitting lasers and may lead to InGaAsN becoming a viable substitute for indium gallium arsenide phosphide (InGaAsP) in 1.3 xcexcm lasers.
Forming an InGaAsN active region is possible using a technique known as molecular beam epitaxy (MBE). MBE uses a nitrogen radical, generated by a plasma source, as a source of active nitrogen species. The purity of the nitrogen is typically high because high purity nitrogen gas is widely available. Further, using MBE, the incorporation efficiency of nitrogen into the epitaxial layer approaches unity. Unfortunately, MBE provides a low growth rate, resulting in a long growth time, and does not scale well, and therefore does not lend itself to high volume production of light emitting devices.
Another technique for producing semi-conductor based light emitting devices is known as organometallic vapor phase epitaxy (OMVPE), sometimes referred to as metal organic chemical vapor deposition (MOCVD). OMVPE uses liquid chemical precursors, through which a carrier gas is passed, to generate a chemical vapor that is passed over a heated semiconductor substrate located in a reactor. Conditions in the reactor are controlled so that the combination of vapors forms an epitaxial film as the vapors pass over the substrate.
Unfortunately, growing high quality InGaAsN using OMVPE is difficult because the purity of the nitrogen precursor (typically dimethylhydrazine (DMHy), [CH3]2NNH2) is difficult to control, and the components that form the InGaAsN alloy are somewhat immiscible. This results in a non-homogeneous mixture where the nitrogen may not be uniformly distributed throughout the layer. Instead, the nitrogen tends to xe2x80x9cclump.xe2x80x9d The alloy composition fluctuations translate into bandgap fluctuations. This causes broadening of the spontaneous emission spectrum and the gain spectrum, which raises laser threshold current.
Furthermore, it is difficult to extract atomic nitrogen from the DMHy molecule, thereby making it difficult to incorporate a sufficient quantity of nitrogen in the InGaAsN film. The ratio of DMHy to arsine (AsH3, the arsenic precursor) must be increased because the arsenic provided by the arsine competes with the nitrogen for the group-V lattice sites. Unfortunately, reducing the proportion of arsine tends to reduce the optical quality of the InGaAsN film.
To incorporate a sufficient quantity of nitrogen in the epitaxial film, extremely high dimethylhydrazine ratios (DMHy:V, where xe2x80x9cVxe2x80x9d represents the total group-V precursor flow rate, comprising DMHy+AsH3) are used during OMVPE growth of InGaAsN. However, even when the DMHy ratio is raised to 90% or greater, the nitrogen component in the film may be negligible ( less than  less than 1%), despite the very high nitrogen content in the vapor. Moreover, the nitrogen content drops even further in the presence of indium, which is a necessary component for a 1.3 xcexcm laser diode quantum well layer. Ideally, for 1.3 xcexcm light emitting devices, the indium content should be about, or greater than, 30%, and the nitrogen content about 0-2%. Without nitrogen, a 1.2 xcexcm wavelength is the longest likely attainable wavelength from a high indium content quantum well (where the maximum indium content is limited by the biaxial compression). The addition of even a very slight amount of nitrogen (0.3% less than [N] less than 2%) leads to a large drop in the bandgap energy that enables the wavelength range to be extended to 1.3 xcexcm and beyond. Nevertheless, it is still difficult to achieve a nitrogen content of [N]xcx9c1%, even when the nitrogen content of the vapor exceeds 90%.
Therefore, it would be desirable to economically mass produce a high optical quality light emitting device having an InGaAsN active layer using OMVPE.
Embodiments of the invention provide several methods for using OMVPE to grow high quality light emitting active regions. In one embodiment, the method comprises placing a substrate in an organometallic vapor phase epitaxy (OMVPE) reactor, the substrate for supporting growth of an indium gallium arsenide nitride (InGaAsN) film, supplying to the reactor a group-III-V precursor mixture comprising arsine, dimethylhydrazine, alkyl-gallium, alkyl-indium and a carrier gas, where the arsine and the dimethylhydrazine are the group-V precursor materials and where the percentage of dimethylhydrazine substantially exceeds the percentage of arsine, and pressurizing the reactor to a pressure at which a concentration of nitrogen commensurate with light emission at a wavelength longer than 1.2 um is extracted from the dimethylhydrazine and deposited on the substrate.
In an alternative embodiment, the method comprises placing a substrate in an organometallic vapor phase epitaxy (OMVPE) reactor, the substrate for supporting growth of an indium gallium arsenide nitride (InGaAsN) film, supplying to the reactor a group-III-V precursor mixture comprising arsine, alkyl-gallium, alkyl-indium and a carrier gas, where the arsine is the group-V precursor material, growing a sublayer of InxGaAs1-x, where x is equal to or greater than 0, discontinuing the group-III precursor mixture, and supplying to the reactor a group-V precursor mixture comprising arsine and dimethylhydrazine where the percentage of dimethylhydrazine substantially exceeds the percentage of arsine.
In another alternative embodiment, the method comprises providing a substrate in an organometallic vapor phase epitaxy (OMVPE) reactor, supplying to the reactor a group-III-V precursor mixture, where the group-III precursor mixture includes alkyl-gallium and alkyl-indium, and the group-V precursor mixture comprises arsine and dimethylhydrazine, and growing a layer of indium gallium arsenide nitride commensurate with light emission at a wavelength longer than 1.2 um over the substrate by minimizing the amount of arsine and maximizing the amount of dimethylhydrazine.
Other features and advantages in addition to or in lieu of the foregoing are provided by certain embodiments of the invention, as is apparent from the description below with reference to the following drawings.